1. Field of the Invention
The present invention relates to sensor circuits (sensor circuits in general, such as magnetic sensors, temperature sensors, and optical sensors), to semiconductor devices built with such sensor circuits integrated thereinto, and to electronic apparatuses provided with such sensor circuits (for example, portable terminals using battery power supplies). More particularly, the present invention relates to a technology for reducing the electric power consumption of such sensor circuits.
2. Description of Related Art
In general, a magnetic sensor circuit is composed of a Hall device that outputs an output voltage commensurate with the strength of a magnetic field, an amplifier that amplifies the output voltage of the Hall device, and a comparator that compares the output voltage of the amplifier with a predetermined reference voltage and outputs the comparison result thus obtained, and is so configured as to output a binary signal (high (H) level or low (L) level) depending on whether the magnetic field in the place where the magnetic field sensor is placed is stronger or weaker than a given level.
To obtain an accurate comparison result commensurate with the magnetic field strength, it is necessary to reduce an offset signal component in the signal outputted from the amplifier and thereby reduce variation in the signal outputted from the amplifier. The offset signal component is mainly caused by an offset signal component (hereinafter a “device offset voltage”) in the output voltage of the Hall device, and an offset signal component (hereinafter an “input offset voltage”) at the input terminal of the amplifier. The device offset voltage is caused primarily by, for example, stress applied to a main body of the Hall device by the package, and the input offset voltage is caused primarily by, for example, variation in characteristics among elements constituting an input circuit of the amplifier.
A magnetic field sensor for reducing the influence by those offset voltages is disclosed in Patent Document 1. That is, a Hall device used in a magnetic field sensor is in general, like a Hall device 1 shown in FIG. 21, formed in the shape of a plate that is geometrically equivalent with respect to four terminals A, C, B, and D. Here, “geometrically equivalent” means that, like the quadrangular Hall device 1 shown in FIG. 21, even when it is rotated 90 degrees (rotated in such a way that the line A-C coincides with the line B-D), the shape thereof remains unchanged from that shown in FIG. 21. For an effective signal component commensurate with the magnetic field strength, a voltage appearing between the terminals B and D when a power supply voltage is applied between the terminals A and C of the Hall device 1 described above is in phase with a voltage appearing between the terminals A and C when a power supply voltage is applied between the terminals B and D; for a device offset voltage, the former is opposite in phase to the latter.
At a first time point, a power supply voltage is applied between the terminals A and C of the Hall device 1 via the switch circuit 2, and a voltage between the terminals B and D is inputted to the voltage amplifier 3. As a result, from the voltage amplifier 3, a voltage V1 commensurate with the sum of the voltage between the terminals B and D and the input offset voltage of the voltage amplifier 3 is outputted. Also, at this first time point, the switch 5 is closed, whereby the capacitor 4 is charged to the voltage V1.
Next, at a second time point, a power supply voltage is applied between the terminals B and D of the Hall device 1 via the switch circuit 2, and a voltage between the terminals C and A is inputted to the voltage amplifier 3 in such a way that a voltage having a polarity opposite to that inputted at the first time point is inputted thereto. As a result, from the voltage amplifier 3, a voltage V2 commensurate with the sum of the voltage between the terminals C and A and the input offset voltage of the voltage amplifier 3 is outputted.
Since the influence of the input offset voltage is the same as that observed at the first time point irrespective of the polarity of the input voltage, the output voltage V2 of the voltage amplifier 3 is a voltage commensurate with the sum of the voltage between the terminals C and A, the voltage having a polarity opposite to that inputted at the first time point, and the input offset voltage.
Also, at this second time point, the switch 5 is opened, whereby the inverting output terminal 3a and the non-inverting output terminal 3b of the voltage amplifier 3 and the capacitor 4 are connected in series between the output terminals 6 and 7. At this point, the charging voltage of the capacitor 4 is kept at the output voltage V1 of the voltage amplifier 3 outputted therefrom at the first time point. A voltage V between the output terminals 6 and 7 (an output voltage of the magnetic field sensor) is obtained as the sum of the voltage V2 at the non-inverting output terminal 3b of the voltage amplifier 3 relative to the inverting output terminal 3a thereof and the voltage −V1 at the terminal 4a of the capacitor 4 relative to the terminal 4b thereof, that is, the voltage V is obtained by subtracting the voltage V1 from the voltage V2. In this way, the voltage V from which the influence of the input offset voltage is offset is obtained as an output voltage of the magnetic field sensor.
A conventionally known magnetic field sensor that can reduce the influence of the input offset voltage of the amplifier as well as the influence of the device offset voltage is disclosed in Patent Document 2. This magnetic field sensor is composed of a Hall device, a switch circuit, a voltage-current converter/amplifier, a capacitor serving as a memory element, a switch, and a resistor.
Patent Document 1: JP-B-3315397
Patent Document 2: JP-A-H08-201491
Patent Document 3: JP-A-H11-131879
In ideal conditions, it can be expected that the magnetic field sensor disclosed in Patent Document 1 performs offset cancellation accurately; in reality, however, the capacitor 4 and the voltage amplifier 3 do not form a perfect differential configuration. Thus, there is a possibility that, for example, a delay (retardation) caused by the capacitor 4 or ripples or noise in the power supply voltage makes it impossible to adequately perform offset cancellation.
On the other hand, with respect to the magnetic field sensor disclosed in Patent Document 2, the following problem arises. This magnetic field sensor requires two voltage-current converter/amplifiers, two capacitors, and four switches. This makes it difficult to reduce the size of a circuit for reducing the influence of the input offset voltage.
Incidentally, since sensor circuits including magnetic field sensors used for detecting continuous movement such as rotation consume a significant amount of electricity during operation, it is preferable to minimize the operation thereof. In particular, in portable apparatuses driven by a battery power supply, it is necessary to reduce the electric power consumption of the sensor circuit as low as possible so as to prolong the battery life.
However, Patent Documents 1 and 2 do not disclose a technology for reducing the electric power consumption.
The applicant of the present invention once proposed a magnetic sensor circuit that can perform measurements with a high degree of accuracy in Japanese Patent Application No. 2005-230781 (claiming priority based on Japanese Patent Application No. 2005-031715). However, the magnetic sensor circuit proposed in this document does not achieve a sufficient reduction in current consumption.